Add directive plugin for jsonic Go implementation

[rjrodger]

Summary

This PR introduces a new directive plugin for the jsonic Go library that enables custom syntax extensions through configurable open/close token sequences. Directives allow users to define custom parsing behaviors that transform parsed content.

Key Changes

• New directive.go module: Core directive implementation with:

  • Action callback type for processing directive content
  • RuleMod struct for configuring grammar rule integration
  • RulesOption for specifying which rules detect open/close tokens
  • DirectiveOptions configuration struct
  • Apply() helper function for easy plugin registration

• New plugin.go module: Main Directive() plugin function that:

  • Registers custom open/close tokens with the jsonic parser
  • Modifies existing grammar rules ("val", "list", "elem", "map", "pair") to detect directive tokens
  • Creates a directive rule that parses content between open/close boundaries
  • Supports optional close tokens (single-value directives when close is omitted)
  • Handles implicit list/map creation within directive boundaries
  • Provides CustomFunc callback for advanced rule customization
  • Implements proper node resolution through replacement chains

• Comprehensive test suite (directive_test.go):

  • Happy path tests for single values, lists, maps, and implicit lists
  • Close token handling with nesting and multiple directives
  • Custom action callbacks (adder, multiplier examples)
  • Edge cases and error conditions
  • Advanced custom rule modifications for shorthand syntax

Notable Implementation Details

• Directives can have optional close tokens; without a close token, they consume a single value
• Multiple directives can share the same close token
• Duplicate open tokens are detected and panic to prevent conflicts
• Counter-based mechanism (dlist, dmap) controls implicit list/map creation within directive boundaries
• Node replacement chain walking ensures correct child node resolution after implicit list creation
• Custom callbacks receive resolved token Tins for advanced grammar modifications
• Default rules can be overridden or disabled via RulesOption

https://claude.ai/code/session_01RiMHE2zSU49VFuFzWygBBF